1. Field of the Invention
The present invention is related to a refresh method for a non-volatile memory, and more particularly to a refresh method which can save operating time and effectively reduce cost.
2. Description of the Prior Art
In the modern society, the tremendous flow of information has become part of people's everyday life. For easy management of information, a memory device for storing information has become a significant subject for development in the information technology (IT) industry. This is especially obvious for a flash memory, for its low power consumption, high working speed, re-programmability, non-volatility and no moving mechanical parts. Thus, the flash memory has been playing an important role among all the memory devices.
The flash memory can be further classified into a floating gate flash memory and a split gate flash memory. Because the floating gate flash memory has merits, such as smaller area for a unit memory cell, faster access speed, and good endurance for Program and Erase commands, the floating gate flash memory is being used in portable electronic devices, computer input/output devices and many consumer devices. In a traditional floating gate flash memory, memory cells in the same P-well are set to be in the same sector, and an insulating layer is set around the P-well, such that the cells in the different sectors can be electrically isolated. The use of the insulation layer can decrease the chip area efficiency of the flash memory; that is to say, the more chip area used by the insulation layer, the less chip area can be used for the memory unit. Therefore, the area of the P-well is usually built to be larger for increasing the chip area efficiency of the memory device.
If all of the memory cells in the same P-well were further divided into multiple sectors of few memory capacities, then the sectors in the same P-well become electrically correlated. In other words, when an Erase command is issued to perform an erase function to a specific sector, the other sectors in the same P-well will be disturbed. When the disturb phenomena accumulates after certain times, data stored in the memory cells will change from “0” to “1” (on the contrary, “1” won't be changed to “0”), and therefore the memory cells need to be refreshed for avoiding the disturb phenomena. The method for refreshing the memory cells in the same P-well is to read out all the data in the same P-well and then written back, but this will need extra memory buffers, and the overall cost is then increased.
Another method in the prior art is to read all the data in the same P-well twice, and compare the data to see whether they are identical; if the two sets of data are not identical, then the memory will perform a refresh process. However, this method is time-consuming, and more complex to implement, and the cost is also increased.